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We can multiply three-digit decimal numbers by a one-digit number using the grid method
Year 6 Unit 10c
What we are learning:
- When we start to multiply numbers with decimals in them it is helpful to return to the expanded method so that we can see what we are doing at each stage in the calculation.
- If we have 15.7 x 6 we can estimate the answer first to know roughly what it will be. If we round 15.7 up to 16 and we know that 16 x 6 is 96 (6 x 10 and 6 x 6 added together), we know that the answer will be slightly smaller than 96.
We can partition the 15.7 into 10, 5 and 0.7 to multiply each part separately like this:
We can now see that 15.7 x 6 is 94.2 which fits our estimate of a number smaller but near to 96.
If we have two decimal places we can use the same method with a hundredths column.
For example, if we have 15.96 x 4 we can estimate the answer by rounding 15.96 to 16 x 4 which is 64. We can then partition 15.96 into 10, 5, 0.9 and 0.06 like this:
When we multiplied the hundredths column we said 4 x 6 hundredths is 24 hundredths. This is the same as two tenths and four hundredths.
When we multiplied the tenths column we said 4 x 9 tenths which is 36 tenths. This is the same as three and six tenths.
Activities you can do at home:
The Activity sheet provides examples for you to work out together. Set them out on a grid like these above to help you get the digits in the correct columns and you multiply and then add the different answers to make the total.
The stages are:
a) Partition the target number carefully and put each part in the right column of the grid
b) Multiply each part – remember that you can only put one digit in each box in the grid!
c) Add the answers to make the total.
Good questions to ask:
How does the grid method help you understand multiplication of decimals?
If your child:
Makes calculation errors when multiplying or adding
Slow down their calculation and ask them to talk it through out loud as they do it – this often helps them identify their own errors and they self correct them as they realise. If they don’t see their error, ask them Are you sure
about that part? This will prompt them to look at it more carefully again.
Also ask them How near is your answer to your estimate? If they don’t match, is the estimate incorrect or is the answer likely to be wrong?